The invention relates to a fuel injection system for mixture compressing, spark plug ignited internal combustion engines employing continuous injection into the induction manifold, in which an air-flow measuring element and an arbitrarily actuatable butterfly valve are disposed in series within the induction manifold. The air-flow measuring element is displaced corresponding to the air flow through the induction tube and against a resetting force. This displacement is transmitted to the movable member of a valve assembly disposed within the fuel line for the purpose of metering out a fuel quantity proportional to the air quantity. The fuel metering process occurs at a constant pressure difference across the valve assembly but the pressure difference may be changed in dependence on motor parameters.
Fuel injection systems of this type serve the purpose of automatically creating a favorable fuel-air mixture suitable for all operational conditions of the internal combustion engine so as to make possible a complete combustion of the fuel and thus to avoid the generation of toxic exhaust components while maintaining the highest possible performance of the internal combustion engine or the least possible fuel consumption. For this reason, the fuel quantity must be metered out very precisely according to the requirement of each operational state of the internal combustion engine and the proportionality between the air quantity and the fuel quantity must be changed in dependence on motor parameters. The laws and regulations affecting the exhaust gas constituents of vehicle engines are becoming more restrictive all the time and make necessary a very precise control of the optimum fuel quantity injected. Thus, for example, the catalyzers which are used, among other things, for exhaust gas detoxification require an air ratio equivalent to an air number .lambda. close to 1.0 in order to achieve the substantially complete transformation of detrimental exhaust components into harmless compounds. It is already known, in fuel injection systems of this type, to alter electromagnetically the pressure difference prevailing at the metering valve and by means of a differential pressure valve but this involves a relatively large constructional expense.